Browsing by Author "Prabhash Mahata"

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Importance of intersystem crossing in the C(3P) + SiH4reaction
(Royal Society of Chemistry, 2020) Mrinmoy Mandal; Prabhash Mahata; Biswajit Maiti
The contribution of intersystem crossing (ISC) in the C(3P) + SiH4reaction that leads to products formation in the singlet electronic state is investigated using a direct dynamics trajectory surface hopping (TSH) method with Tully's fewest switches algorithm. Interestingly, in contrast to the O(3P) + SiH4reaction with no ISC effect, for the title reaction we observed ∼7% product formation through ISC despite weak spin-orbit coupling interactions (less than 25 cm−1) between the ground singlet and triplet states. This is presumably because of the topological differences in the potential energy surfaces of the two reactions at the entrance channel. The O(3P) + SiH4reaction follows either an addition reaction (with shallow attractive potential and a late singlet-triplet crossing) or a direct abstraction pathway with singlet-triplet crossing at near or after the top of the barrier making ISC ineffective. On the other hand, an insertion mechanism is exclusively followed by the C(3P) + SiH4reaction with no entrance barrier to the reaction in the triplet state. The triplet insertion complex initially formed (3H3SiCH) can go to the singlet state through ISC due to the fact that the triplet-singlet crossing is accessed several times during the course of the reaction. Our computed overall product angular distributions for H and H2elimination channels are found to be broad and flat or nearly isotropic in nature indicating the formation of stable intermediate complexes, which corroborates the most recent crossed molecular-beam study. © the Owner Societies 2020.
Photodissociation dynamics of HN3 at 248 nm: a trajectory surface hopping study
(Springer, 2023) Subhendu Ghosh; Prabhash Mahata; Biswajit Maiti
The photodissociation dynamics of hydrazoic acid (HN3) at 248 nm has been studied using a quasi-classical variant of the trajectory surface-hopping (TSH) method in conjunction with Tully’s fewest switches algorithm. Trajectories were integrated on-the-fly at the MRCIS(8,9)/6-31++G(d,p) level of theory. Analysis of our trajectory simulation reveals that N2 (X ~ 1Σg+) + NH (a 1Δ) as the primary major products contributing ~95% of the overall product formation with N3 (X~ 2Π g) + H(2S) as the minor products contributing the rest ~5%. No internal conversion (IC) from the first excited state S1 to the ground state S0 was observed in the photodissociation of HN3 from its first excited singlet state (S1). Intersystem crossing (ISC) from the first excited singlet state S1 to the lowest triplet state T1 was predicted to be inefficient based on the calculated weak spin-orbit interactions between the states. Graphical abstract: Synopsis We have studied the photodissociation dynamics of hydrazoic acid (HN3) using the trajectory surface-hopping (TSH) method. Our trajectory simulation reveals that N2 (X ~ 1Σg+) + NH (a 1Δ) as the primary major products contributing ~95% of the overall product formation with N3 (X~ 2Π g) + H(2S) as the minor products contributing the rest ~5%. No internal conversion (IC) was observed, and intersystem crossing (ISC) was predicted to be inefficient.[Figure not available: see fulltext.] © 2023, Indian Academy of Sciences.
Photodissociation Dynamics of Methyl Hydroperoxide at 193 nm: A Trajectory Surface-Hopping Study
(American Chemical Society, 2021) Prabhash Mahata; Biswajit Maiti
The photodissociation of methyl hydroperoxide (CH3OOH) at 193 nm has been studied using a direct dynamics trajectory surface-hopping (TSH) method. The potential energies, energy gradients, and nonadiabatic couplings are calculated on the fly at the MRCIS(6,7)/aug-cc-pVDZ level of theory. The hopping of a trajectory from one electronic state to another is decided on the basis of Tully’s fewest switches algorithm. An analysis of the trajectories reveals that the cleavage of the weakest O-O bond leads to major products CH3O(2E) + OH(2Π), contributing about 72.7% of the overall product formation. This OH elimination was completed in the ground degenerate product state where both the ground singlet (S0) and first excited singlet (S1) states become degenerate. The O-H bond dissociation of CH3OOH is a minor channel contributing about 27.3% to product formation, resulting in products CH3OO + H. An inspection of the trajectories indicates that unlike the major channel OH elimination, the H-atom elimination channel makes a significant contribution (∼3% of the overall product formation) through the nonadiabatic pathway via conical intersection S1/S0leading to ground-state products CH3OO(X2A″) + H(2S) in addition to adiabatic dissociation in the first excited singlet state, S1, correlating to products CH3OO(12A′) + H(2S). The computed translational energy of the majority of the OH products is found to be high, distributed in the range of 70 to 100 kcal/mol, indicating that the dissociation takes place on a strong repulsive potential energy surface. This finding is consistent with the nature of the experimentally derived translational energy distribution of OH with an average translational energy of 67 kcal/mol after the excitation of CH3OOH at 193 nm. © 2021 American Chemical Society
Soluble and highly fluorescent conjugated polymer network: non-oxidative reversible doping, cell imaging and anticancer activity
(Royal Society of Chemistry, 2023) Neelam Gupta; Swapan Maity; None Anamika; Ravi Prakash Behere; Prabhash Mahata; Biswajit Maiti; Pralay Maiti; Biplab Kumar Kuila
Herein, we report two soluble and highly fluorescent triazine-based conjugated polymer networks (TCPNs) by conjugating a thiophene (donor) unit with a triazine ring (acceptor) through a phenyl linker. The 3-substituted thiophene moiety with alkyl and oligoethylene glycol side chains is rationally introduced into the polymeric network to improve its solubility and the effect of the side chain polarity on its optical, band gap, conductivity and biological activities was also investigated. When both TCPNs were reacted with a Lewis acid and base respectively, significant optical switching and a conductivity change were observed, indicating efficient non-oxidative doping/de-doping. Both TCPNs exhibit remarkable stability after several switching cycles from their neutral to doped states and vice versa. Doping of the TCPN through the addition of a Lewis acid is also studied via DFT, which clearly shows a narrowing of the band gap. To the best of our knowledge, this may be the first demonstration of the non-oxidative doping of a triazine-based CPN to control the conjugation, color and conductivity. Furthermore, these TCPNs show gelation after doping with a Lewis acid, resulting in the formation of a conducting gel. It is also observed that both TCPNs can enter cells to simultaneously exhibit abilities of fluorescence imaging and cancer cell inhibition. Thus, the described study showcases the potential scope of tuning the optical properties of triazine-based conjugated polymer networks for electronic and optoelectronic applications, as well as the probable use of these polymers for cell imaging and chemotherapeutic applications. © 2023 The Royal Society of Chemistry.
Sulfuric acid decomposition chemistry above Junge layer in Earth's atmosphere concerning ozone depletion and healing
(Springer Nature, 2019) Montu K. Hazra; Sourav Ghoshal; Prabhash Mahata; Biswajit Maiti
Sulfuric acid (H2SO4) is the seed molecule for formation of stratospheric sulfate aerosol layer that assists ozone depletion by activation of halogen species. The impact of increased stratospheric sulfate aerosols due to large volcanic eruptions and possible side effect claimed in the geoengineering scheme of global climate using man-made injected stratospheric sulfate aerosols is ozone depletion. Given that both volcanic eruptions and geoengineering scheme are ultimately connected with increased upper stratospheric concentrations of H2SO4, here we show by theoretical approach that the pressure-independent H2SO4 + O(1D) insertion/addition reactions via barrierless formation of peroxysulfuric acid (H2SO5) or HSO4 + OH radicals or sulfur trioxide (SO3) + hydrogen peroxide (H2O2) molecules are the potential routes towards H2SO4 loss above the stratospheric sulfate aerosol layer, and for the regeneration or transportation of consumed lower-middle stratospheric OH radical in the upper stratosphere at the cost of O(1D)/ozone. © 2019, The Author(s).
Trajectory surface-hopping study of 1-pyrazoline photodissociation dynamics
(American Institute of Physics Inc., 2022) Prabhash Mahata; Akshaya Kumar Rauta; Biswajit Maiti
The photodissociation dynamics of 1-pyrazoline has been studied from its first excited electronic state (S1) using the Direct Dynamics Trajectory Surface-Hopping method in conjunction with Tully's fewest switches algorithm at the CASSCF(8,8)/6-31G∗ level of theory. After excitation of the molecule into the Franck-Condon region of the first excited state, S1, the molecule hops to the ground (S0) state quickly. The dissociation of one of the C-N bonds initially starts in the first excited state. Then, the molecule comes to the ground state (S0) via S1/S0 conical intersections, followed by complete dissociation in the ground state. Two different conical intersections are identified between the first excited singlet (S1) and the ground (S0) electronic states. One primary and three secondary dissociation channels are observed from our dynamics calculations of photodissociation of 1-pyrazoline that are in accord with the experimentally observed channels. After internal conversion to the ground electronic state (S0), the molecule dissociates to N2 and trimethylene biradical as the primary dissociation products. The trimethylene biradical then rearranges, leading to three secondary dissociation channels, N2 + cyclopropane, N2 + CH2 + C2H4, and N2 + CH3CHCH2. The major products formed from the trimethylene biradical in the secondary process is cyclopropane contributing about 78% of the overall products formation along with ∼12% propene and the rest ∼10% methylene (CH2) with ethene (C2H4). © 2022 Author(s).